Thirty-one of the longest available streamflow records for New Zealand were analysed to see whether shifts in flood and low-flow regime occurred in 1977/1978 corresponding to a shift in phase of the Interdecadal Pacific Oscillation. Annual peak flows, and annual minimum flows averaged over 28 days, for two periods, 1947-1977 and 1978-1999, were compared using box plots and statistical tests. The plots and the tests show that a decrease of flood size has occurred since 1978 in the Bay of Plenty region of the North Island, and that increases in flood size and low-flow magnitude have occurred in the South Island for most rivers with headwaters draining from the main divide of the Southern Alps and Southland. For much of the North Island apart from the Bay of Plenty, and the north of the South Island, no consistent pattern of shifts was identified.

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The Southern Oscillation index (SOI) can yield information about subsequent streamflows on the South Island of New Zealand; however, the relationship between streamflow and SOI may be nonlinear and heteroscedastic. To deal with such difficulties, the conditional probability of streamflow's being below a critical magnitude, given a prior observation of SOI, can be considered a random variable, with its probability density function (pdf) estimated by Bayesian analysis of existing observations of streamflow and SOI. The conditional pdf can yield a probabilistic forecast of critically low streamflows given a precursor value of SOI. In the Clutha River basin of New Zealand, average austral summer inflow to the headwater lakes has an unconditional probability of nonexceedance of 360 m3/s of approximately 17%. However, during a moderate La Niña, average austral spring SOI = 12, the conditional probability of nonexceedance has an expected value of 55%. For an El Niño with a spring SOI of −12, the expected probability of nonexceedance is 18%; for a neutral SOI of 0, the expected probability is 10%. Thus, the probability of nonexceedance of seasonal streamflow can be seen to vary by a factor of more than 5 as a function of the SOI.

The two-parameter EV1 distribution adequately describes New Zealand's flood series. Contour maps of Q/A°-& and <2 10 r/2 are presented, where £) is the mean annual flood, A is the basin area and Q 1Q0 is the 1% annual exceedance probability flood. The maps are based directly on measured discharge series from a large sample of river recording stations. Thus when basins are ungauged, or have just a short record, an estimate of a design flood Q T with specified annual exceedance probability (1/T) can be obtained using -map estimates of ~Q/A 0 -8 and Q 100 /<2, without having first to estimate rainfall statistics for the basin, a particularly difficult task in sparsely instrumented mountainous areas. These maps succinctly summarize a great deal of hydrological information and permit improved flood frequency estimates. Cartes des caractéristiques statistiques des crues pour l'analyse régionale des fréquences des crues en Nouvelle-ZélandeRésumé La distribution à deux paramètres EV1 représente les séries de crues de la Nouvelle-Zélande d'une façon adéquate. Dans les cartes avec courbes des valeurs "Q/A 0 -8 et Q 1Q0 /Q, Q représente la moyenne des crues annuelles, A est la surface du bassin et <2 100 est la crue annuelle de probabilité 1% au dépassement. Les cartes sont basées directement sur des séries de débits mesurés d'un large échantillon de stations enregistreuses de débit. Ainsi, quand les stations n'ont pas été jaugées ou présentent une durée d'enregistrement limité, une estimation de la crue de projet Q T avec une probabilité spécifique au dépassement annuel (1/T) peut être obtenue en utilisant les estimations cartographiques de Q/A 0 -8 et Q lQ0 /Q, sans avoir à faire d'abord l'estimation des caractéristiques statistiques des précipitations pour le bassin, tâche particulèrement difficile dans les régions montagneuses où les instruments enregistreurs sont peu nombreux. Ces cartes résument succinctement une grande partie de l'information hydrologique et ont amélioré les estimations des fréquences de crues.